JP2005058309A - Cone beam x-ray ct device and phantom used for the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a C arm type cone beam X-ray CT device capable of suitably obtaining and correcting mechanical manufacture errors, or the like, due to artifacts in the case that a geometrical parameter is biased depending on the range of a rotation angle. <P>SOLUTION: The cone beam X-ray CT device is provided with: error calculation means (304 and 305) for calculating the error of the rotation center axis of rotary movement and the error between the set values of the rotation start angle and rotation speed of the rotary movement and actual values by using the reconstituted image of a phantom including a plurality of wires arranged roughly in parallel to the rotation center axis; and a correction means (205) for correcting three-dimensional X-ray CT images by using the calculated errors. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an X-ray CT apparatus, and more particularly to a technique for suitably correcting a mechanical manufacturing error that causes an artifact.

The cone-beam X-ray CT system is the same as the X-ray source and a two-dimensional X-ray detector (including a combination of an X-ray image intensifier and a TV camera) placed opposite to the X-ray source. An X-ray fluoroscopic image of the subject located on the rotation center axis is photographed with a two-dimensional detector while rotating on the circular orbital plane of the rotation center, and image reconstruction calculation is further performed based on the X-ray fluoroscopic image. By doing so, a three-dimensional X-ray CT image is obtained.
A typical example of an image reconstruction calculation algorithm for a cone beam X-ray CT apparatus using such a two-dimensional X-ray detector is the Feldkamp method (see Non-Patent Document 1, for example).

"Practical Cone-Beam Algorithm; L.A.Feldkamp, et al .; J.Optical Society of America, A / Vol. 1 (6), (1984), pp.612-619".

  In addition, in the cone beam X-ray CT apparatus, in order to obtain a 3D X-ray CT image free from artifacts due to mechanical manufacturing errors, the coordinate axis on the image of the 2D X-ray detector and the actual rotation center axis Finding the difference in angle (hereinafter referred to as coordinate torsion angle), the deviation of the rotation center axis from the design position, etc. as parameters (hereinafter referred to as geometric parameters), and performing the calculation while correcting in the image reconstruction calculation However, there are conventional techniques described in Patent Document 1 and Patent Document 2 as methods for obtaining geometric parameters.

Japanese Unexamined Patent Publication No. 9-17330 JP 2000-201918

  In the prior art of Patent Document 1, a phantom in which a plurality of small metal spheres having a large X-ray absorption coefficient are placed on a rod-shaped support is arranged in parallel to the rotation center axis, and 360-degree rotation photography is performed. This is a method for determining a coordinate torsion angle or the like using an elliptical orbit drawn on a dimensional X-ray projection image. On the other hand, in the prior art described in Patent Document 2, a phantom with a thin metal wire with a large X-ray absorption coefficient stretched on a rod-like support is placed in parallel to the rotation center axis, and rotated 360 degrees. This is a method for obtaining the position of the rotation center axis with high accuracy by using the sharpness of the metal line in the configuration image as an evaluation function so that the evaluation function is optimized.

  The present inventor has found the following problems as a result of studying the above prior art. In other words, a C-arm is sometimes used to support an X-ray source or a two-dimensional X-ray detector in a cone-beam X-ray CT apparatus, but a cone-beam X-ray CT apparatus using a C-arm has an X-ray on one side. It has a structure with a source and a two-dimensional X-ray detector on the other side. In that case, because the weight of both ends of the C-arm is uneven, rotate the C-arm from the lowest position to lift the heavy two-dimensional detector until it comes to the top. Until it is lowered to the opposite side, there is a problem that the geometric parameter is different due to the deflection of the C-arm. If the reconstruction calculation is performed assuming that the geometric parameters are exactly the same in the first half and the second half rotated 360 degrees, there is a problem in that a positional shift occurs, resulting in image artifacts.

  An object of the present invention is to provide a C-arm type cone beam X-ray CT apparatus capable of suitably obtaining and correcting a mechanical manufacturing error or the like that causes an artifact when a geometric parameter is biased depending on a rotation angle range. Is to provide.

  According to the present invention, the X-ray source and the two-dimensional X-ray detector arranged opposite to the X-ray source are arranged on the rotation center axis while being rotated on the circular orbital surface having the same rotation center. A three-dimensional X-ray CT image is generated and displayed by photographing an X-ray fluoroscopic image of the subject with the two-dimensional detector and performing an image reconstruction operation based on the photographed fluoroscopic image. In the cone beam X-ray CT apparatus, a plurality of rotation error center rotation errors and errors with the rotation start angle and rotation speed set value are arranged substantially parallel to the rotation center axis. A cone beam X-ray CT apparatus comprising: an error calculating means for calculating using a reconstructed image of a phantom including a wire; and a correcting means for correcting the three-dimensional X-ray CT image using the calculated error Is provided.

  Further, according to the present invention, each error corresponds to a first rotation angle range and a second rotation angle range, and each error is calculated by the error calculation means, and the first rotation angle The error calculated corresponding to the range is used to correct the three-dimensional X-ray CT image corresponding to the first rotation angle range by the correction means, and calculated corresponding to the second rotation angle range. The corrected error is used for correcting the three-dimensional X-ray CT image corresponding to the second rotation angle range by the correcting means, and the corrected first and second three-dimensional X-ray CT images are used. And a superimposing means for superimposing the three-dimensional X-ray CT images in the first and second rotation angle ranges corrected based on the calculated misregistration. A cone-beam X-ray CT apparatus is provided.

According to the present invention, there is provided a cone beam X-ray CT apparatus, wherein the number of divisions of the angle range for dividing each error is three or more.
According to the present invention, the angle (hereinafter referred to as the twist angle) between the direction of the projection line of the rotation center axis to the two-dimensional detector and the direction of the projection line on the design of the two-dimensional detector. There is provided a cone beam X-ray CT apparatus, comprising: means for calculating a torsion angle of the X-ray fluoroscopic image using the calculated twist angle.

  According to the present invention, the phantom including the plurality of wires arranged substantially in parallel is arranged so that the plurality of wires are arranged vertically and horizontally when viewed from a cross section perpendicular to the direction of the wires. A characteristic phantom is provided.

  According to the present invention, a C-arm type cone beam X-ray CT apparatus capable of suitably obtaining and correcting a mechanical manufacturing error or the like causing an artifact when the geometric parameter is biased depending on a rotating angle range. Can be provided.

Hereinafter, the present invention will be described together with embodiments (examples) of the present invention with reference to the drawings.
Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiment of the invention, and the repetitive description thereof is omitted.

  FIG. 1 is a block diagram showing a schematic configuration of a C-arm cone beam X-ray CT apparatus according to an embodiment of the present invention. C-arm type cone-beam X-ray CT system mainly performs 3D reconstruction based on measurement unit 50, measurement unit control means 100 that controls measurement unit 50, and measurement data obtained by measurement unit 50 3 The geometric parameters for calculating the geometric parameters used for correction in the three-dimensional reconstruction in the three-dimensional reconstruction means 200, numerically representing the error in mechanical fabrication of the dimension reconstruction means 200 and the measuring unit 50 The calculation means 300 is configured. In addition, an image collecting unit 110 that collects and records the X-ray transmission image obtained by the measurement unit 50 and an image display unit 210 that displays the reconstructed image obtained by the three-dimensional reconstruction unit 200 are provided. Yes.

  First, the measurement unit 50 includes a bed 4 on which the subject 3 is placed, an X-ray tube 1 that irradiates the subject 3 placed on the bed 4 with X-rays, and an X-ray tube 1 installed on the X-ray tube 1. A collimator 2 that controls the direction of X-ray irradiation, and an X-ray tube 1 that is disposed opposite to the subject 3 on the opposite side of the X-ray tube 1 and is irradiated from the X-ray tube 1 and passes through the subject 3 An X-ray image intensifier 5 that captures a line transmission image and converts it into a visible light image, a TV camera 6 that captures a visible light image converted by the X-ray image intensifier 5, and an X-ray irradiation side ( A C-arm 10 that mechanically connects the X-ray tube 1 and collimator 2) and the X-ray imaging side (X-ray image intensifier 5 and TV camera 6), and a C-arm holder 11 that holds the C-arm 10; The C-arm holder 11 is attached to the ceiling, and the entire C-arm can be moved by moving the ceiling support 12 on the rail. A ceiling rail 13 is provided. However, in FIG. 1, 8 is a rotation center axis, 7 is a rotation orbit plane (midplane) which is a plane including the focal path of the X-ray source, and 9 is an intersection of the rotation center axis 8 and the rotation orbit plane 7 (hereinafter referred to as rotation). Called the center).

  The measurement unit control means 100 includes a C-arm rotation control means 101 that controls rotation of the C-arm 10 around the rotation center axis 8 (rotation around the rotation center axis 8 is referred to as propeller rotation), and ceiling An imaging system position control means 102 that controls the position of the support 12 on the ceiling rail 13 and two-dimensionally controls the position of the entire C arm, and ON / OFF of the tube current flowing through the X-ray tube 1 is controlled. X-ray irradiation control means 103, subject position control means 104 for controlling the position of the subject by controlling the position of the bed 4, and imaging of X-ray transmission images by the X-ray image intensifier 5 and the TV camera 6 An imaging system control means 105 and the like for controlling are provided.

  The three-dimensional reconstruction means 200 is a three-dimensional X-ray CT image obtained by performing image reconstruction calculation based on the X-ray fluoroscopic image measured by the measurement unit 50 and the measurement unit control unit 100 and recorded in the image acquisition unit 110. The image processing apparatus includes a preprocessing unit 201, an image distortion correction unit 202, a coordinate torsion angle correction unit 203, a filtering unit 204, a back projection unit 205, and a three-dimensional image overlay unit 206. They will be described in detail below in order.

  The pre-processing means 201 is a known means for converting the X-ray transmission image collected and recorded by the image collecting means 110 into a distribution image of X-ray absorption coefficients. In the present embodiment, first, natural logarithmic transformation calculation is performed on each pixel data of an X-ray transmission image of air that has been imaged in advance without the subject 3 and the bed 4 being placed in the field of view. Next, a natural logarithmic conversion operation is performed on each pixel data obtained by capturing an X-ray transmission image with the subject 3 placed on the bed 4. By taking the difference between the two images, an X-ray absorption coefficient distribution image of the subject 3 and the bed 4 is obtained.

The image distortion correction means 202 is a known means described in Japanese Patent Laid-Open No. 8-24248, and corrects image distortion in the distribution image of the X-ray absorption coefficient. This image distortion is an image distortion generated when an X-ray transmission image is converted into a visible light image by the X-ray image intensifier 5, and an image distortion correction table stored in an image distortion correction table storage unit 302 described later. Based on the above, the image distortion of the distribution image of the X-ray absorption coefficient obtained by the preprocessing means 201 is corrected.
The coordinate torsion angle correcting unit 203 is an image rotating unit that corrects an angle difference (coordinate torsion angle) between the coordinate axis on the image of the two-dimensional X-ray detector and the actual rotation center axis.

  Here, the coordinate twist angle will be described with reference to FIG. 20 is a Hall chart, 21 is a projection of the rotation center axis 8, 7 is a rotation orbit plane, 22 is a vertical axis on the Hall chart, 23 is a horizontal axis on the Hall chart, and 24 is a coordinate twist angle b. The hole chart 20 is a phantom used for acquiring calibration data in an image distortion correction table generation unit 301 to be described later, and is a metal plate in which minute points (pinholes) are spaced on equidistant lattice points. The Hall chart 20 is fixed to the front of the X-ray image intensifier 5 to collect the Hall chart distortion projection image, but the vertical axis 22 on the Hall chart and the projection 21 of the rotation center axis 8 or the horizontal axis on the Hall chart When 23 and the rotational track surface 7 are shifted by the coordinate twist angle b, the image after the image distortion correction by the image distortion correction means 202 is shifted by the twist angle b. This coordinate twist angle b is obtained by coordinate twist angle calculation means 306 described later, and the X-ray image is corrected in the direction opposite to the coordinate twist angle b. However, in the present embodiment, the X-ray image intensifier 5 and the TV camera 6 are used as the X-ray imaging system. However, when replacing with a two-dimensional X-ray detector using a TFT element or the like, the X-ray detector The deviation of the mounting angle becomes the coordinate twist angle b.

The filtering unit 204 is a known unit that performs a filtering process in X-ray CT image reconstruction.
The back projection means 205 is a known means for performing back projection processing on the image data after filtering using the felt-kamp method described in Document 1. However, in this embodiment, as described in [0007], it is considered that the geometric parameters are different between the first half of rotation and the second half of the rotation, and back projection is performed using each X-ray transmission image using each geometric parameter. Process.

  The three-dimensional image superimposing unit 206 is a unit that adds the three-dimensional X-ray images that are back-projected by the back projection unit 205 with the geometric parameters of the first half of rotation and the second half of rotation. In this embodiment, the addition generates a three-dimensional X-ray image using two-dimensional X-ray projection data from the direction of 200 degrees in the first half and the second half. The generated three-dimensional X-ray image is displayed by the image display means 210.

  Next, the geometric parameter calculation unit 300 prepares a mechanical manufacturing error of the cone beam X-ray CT apparatus to be incorporated as a correction when performing image reconstruction by the three-dimensional reconstruction unit 200 as a geometric parameter. Distortion correction table generation means 301, image distortion correction table storage means 302, rotation trajectory plane (midplane) reconstruction means 303, rotation center axis position and rotation angle pitch calculation means 304, rotation start angle and overlay reference Position calculation means 305 and coordinate torsion angle calculation means 306 are provided. They will be described in detail below in order.

  The image distortion correction table generation unit 301 is a known unit described in Japanese Patent Laid-Open No. 8-24248, and generates an image distortion correction table used by the image distortion correction unit 202. First, the Hall chart 20 is fixed to the front surface of the X-ray image intensifier 5, and a Hall chart distortion projection image is taken. Hall chart image distortion correction table in which pre-processing means 201 is applied to the hole chart distortion projection image, the projection position of the pinhole-shaped hole is detected, and the pinholes of the corrected hole chart projection image are arranged on the lattice points Calculate

  The image distortion correction table storage unit 302 stores the image distortion correction table calculated by the image distortion correction table generation unit 301 on a magnetic disk or the like, and when the image distortion correction unit 202 corrects the distortion of the projection image data, This is a well-known means that makes it possible to read an image distortion correction table from a disk.

The rotational trajectory plane reconstruction means 303 is means for generating a reconstructed image limited to a plane within the rotational trajectory plane (midplane) 7 in the back projection means 205.
Here, the rotational orbit plane (midplane) 7 is defined as a plane including the focal path of the X-ray source in [0016], but is also a plane characterized by the following definition. Focus on the point (x, y, z) on the subject. Since the distance between the X-ray source and the point (x, y, z) changes due to the rotation, the projection point on the two-dimensional detector surface generally moves in the vertical direction as can be seen from the perspective principle. However, on the plane that includes the X-ray source and is perpendicular to the center axis of rotation, the projection point on the two-dimensional detector plane is up and down even if the distance between the X-ray source and the point (x, y, z) changes due to rotation Don't move on. Even if it is defined as such a surface, the rotating raceway surface (midplane) 7 is characterized.

  The rotation center axis position and rotation angle pitch calculation means 304 is based on the image generated by the rotation track plane reconstruction means 303 and the actual position and rotation angle pitch of the rotation center axis used by the back projection means 205. It is a means for calculating a value. Thus, when the rotation center axis and the rotation angle pitch are not as designed values, geometric parameters for correction are obtained and can be used for the back projection process in the back projection unit 205.

The rotation start angle and overlay reference position calculation unit 305 is based on the image generated by the midplane reconstruction unit 303, and the actual value of the rotation start angle and overlay reference position used by the 3D image overlay unit 206. Is a means for calculating. As a result, when the rotation start angle and the overlay reference position are not as designed values, geometric parameters for correction are obtained, and the 3D image superimposing means 206, for example, in the first half of rotation and the second half of rotation, respectively. It can be used when superimposing 3D X-ray images.
The coordinate twist angle calculation means 306 is a means for calculating the magnitude of the coordinate twist angle b used in the coordinate twist angle correction means 203.

  In the cone beam X-ray CT apparatus using the C-arm configured as described above, the actual dimensions are, for example, the distance between the focal point of the X-ray tube 1 and the rotation center 9 is 800 mm, and the rotation center 9 and the X-ray image intensifier 5 The distance from the X-ray input surface is 400 mm. The C arm 10 is so-called propeller rotated around the subject 3 (rotation center axis 8) by the C arm rotation control means 101. In this embodiment, the total rotation angle of the C-arm is not completely 360 degrees, and the X-ray image intensifier 5 and the TV camera 6 are initially in the direction of the left hand (−100 °) of the subject 3 and the ceiling Moves in the right direction (+ 100 °) of the subject 3 through the direction (0 °). A typical example of the rotational speed of the C-arm 10 is 40 degrees per second, and the scan time is 5 seconds. In this embodiment, 200 degree rotation shooting is performed continuously, but the tendency of mechanical manufacturing errors between the first half 100 degrees and the second half 100 degrees differs from the ceiling direction. It is assumed that they are different.

  Next, the operation during photographing will be described below. When shooting is started, first, the C-arm rotation control means 101 starts rotating the C-arm 10. After the rotation acceleration period, the X-ray irradiation control means 103 generates X-rays from the X-ray tube 1, and the imaging system control means 105 starts capturing a visible light image by the television camera 6. X-rays irradiated from the X-ray tube 1 pass through the subject 3 and then enter the X-ray image intensifier 5. The X-ray transmission image is converted into a visible light image by the X-ray image intensifier 5 and taken into the television camera 6. The TV camera 6 converts the X-ray transmission image into a video signal, performs A / D conversion, and the obtained digital image is recorded in the image collecting means 110. The standard scanning mode of the television camera 6 is, for example, 30 frames per second and 512 scanning lines. The nominal rotation angle pitch is 1.33 degrees, and 150 X-ray images are acquired in 5 seconds. When the 200-degree rotation imaging is completed, the X-ray irradiation control means 103 ends the generation of X-rays from the X-ray tube 1, and the C-arm rotation control means 101 stops rotating. On the other hand, the 3D reconstruction means 200 reads out a digital image from the image collection means 110 in parallel with or after imaging, and reconstructs a 3D X-ray image of the subject 3 by a 3D reconstruction process described later. The calculation is performed and displayed on the image display means 210. However, the image display means 210 can also directly display the X-ray image recorded in the image collection means 110.

  Next, the operation of each unit in the three-dimensional reconstruction unit 200 and the geometric parameter calculation unit 300 will be described. However, as described above, the preprocessing unit 201, the image distortion correction unit 202, the filtering unit 204, the back projection unit 205, the image distortion correction table generation unit 301, and the image distortion correction table storage unit 302 are well-known units. A coordinate torsion angle correcting means 203, a three-dimensional image superimposing means 206, a midplane reconstruction means 303, a rotation center axis position and rotation angle pitch calculating means 304, a rotation start angle and overlapping reference position calculating means 305, The coordinate twist angle calculation means 306 includes the features of the present invention. Therefore, in the following description, the three-dimensional reconstruction unit 200 and the geometric parameter calculation unit 300 will be described with emphasis on the unit including the features of the present invention.

  In order to calculate the geometric parameter in the geometric parameter calculation means 300, a phantom shown in FIG. 3 is used. FIG. 3 is a view of the subject 3 seen from the foot direction, and 30 is a wire phantom. For simplicity, only the X-ray tube 1, the collimator 2, the bed 4, the X-ray image intensifier 5 and the TV camera 6 are shown, and the others are omitted. In the present embodiment, the wire-like phantom 30 stretches a plurality of wires in parallel to the rotation center axis, and generates a cross-sectional image parallel to the rotation orbit plane (midplane) 7 of the three-dimensional X-ray image. Are arranged so that their positions are aligned vertically and horizontally. However, in FIG. 3, an example is shown on the assumption that wires are arranged in 5 × 5. Based on the calibration data collected using the wire phantom 30 and the hole chart 20 described in FIG. 2, the geometric parameter calculation means 300 performs a geometric parameter calculation process.

Next, FIG. 4 shows a flowchart of the geometric parameter calculation processing in the geometric parameter calculation means 300. Each step will be described in turn. However, in the following, in the present embodiment, the step numbers are indicated as S20, S301, etc. in the figure.
(Step 20) First, the Hall chart 20 is fixed to the front surface of the X-ray image intensifier 5 and rotational imaging is performed, and Hall chart distortion projection images at each angle, for example, 150 images are collected.
(Step 301) Next, the image distortion correction table generating means 301 calculates a hole chart image distortion correction table based on the 150 hole chart image distortion projection images obtained in (step 20).
(Step 302) Next, the hole chart image distortion correction table calculated in (Step 301) is stored in the image distortion correction table storage means 302. The Hall chart image distortion correction table stored here is used when the image distortion correction unit 202 corrects the image distortion of the fluoroscopic image.

(Step 30) The wire-shaped phantom shown in FIG. 3 is arranged so that the direction of the wire is the same as the direction of the central axis of rotation, and the odd-numbered × odd-numbered wire in the center (3 × 3 In the case of a wire-shaped phantom, the one in the second row and the second row, and in the case of a 5 × 5 wire-shaped phantom, in the third row and the third row) Arrange it so that it is placed on the top, and rotate the C-arm type cone-beam X-ray CT device to obtain an X-ray fluoroscopic image.
(Step 304) The rotation center axis position and rotation angle pitch calculation means 304 calculates actual values (differences from design values) of the rotation center axis position and rotation angle pitch. Details of the specific processing of the calculation will be described later with reference to FIG. The actual rotation center axis position and rotation angle pitch obtained here are used when the back projection unit 205 performs back projection processing of the actual X-ray fluoroscopic image.

(Step 305) The rotation start angle and overlay reference position calculation means 305 calculates the actual value (difference from the design value) of the rotation start angle and overlay reference position. Details of the specific processing of the calculation will be described later with reference to FIG. The rotation start angle and superposition reference position obtained here are used when the three-dimensional image superimposing means 206 superimposes the reconstructed images at 100 degrees each in the first half and the second half.
(Step 306) The coordinate twist angle calculation means 306 calculates the coordinate twist angle described in [0021]. Details of the specific processing of the calculation will be described later with reference to FIG. The coordinate twist angle obtained here is used when the coordinate twist angle is corrected by the coordinate twist angle correcting means 203.

In step 304 and step 305 of the geometric parameter calculation process shown in FIG. 4 above, reconstruction operations are limited to projection data on the rotating orbital plane (midplane) 7 in order to shorten the calculation time. Process and use the result. Next, the procedure of reconstruction calculation processing (rotation track plane reconstruction calculation processing) limited to the rotation track surface (midplane) 7 will be described with reference to the flowchart of FIG.
(Step 201) Pre-processing is performed by the pre-processing means 201 using projection data limited to the rotation orbit plane (midplane) 7.

(Step 202) The image distortion correction unit 202 performs image distortion correction processing on the data that has been pre-processed in the step 201 and limited to the rotational orbit plane (midplane) 7.
(Step 204) The filtering means 204 performs filtering processing on the data that has been subjected to the image distortion correction processing limited to the rotation orbit plane (midplane) 7 in (Step 202).
(Step 205) The backprojection means 205 performs backprojection processing on the data subjected to the filtering processing limited to the rotational orbital plane (midplane) 7 in (Step 204).

  A reconstructed image on the rotating orbital plane (midplane) 7 is obtained by the above steps (step 201) to (step 205). However, in this embodiment, it is assumed that the tendency of the mechanical manufacturing error is different between the first half 100 degrees and the second half 100 degrees of the C-arm propeller rotation. Therefore, (Step 304) and (Step 305) The rotational trajectory plane reconstruction image is obtained from each projection data in the latter half, and the geometric parameter is obtained by each.

  Next, FIG. 6 shows a flowchart of rotation center axis position calculation and rotation angle pitch calculation processing in the rotation center axis position and rotation angle pitch calculation means 304. Each step will be described in turn. However, since the rotation center axis position and the rotation angle pitch are calculated in the same procedure in the first half of the rotation and the second half of the rotation, only the first half of the rotation will be described, and the second half of the rotation will be omitted.

(Step 401) First, projection data for the first half of the rotation is selected from the wire-like phantom projection images collected in (Step 30) of FIG.
(Step 402) The initial value of the rotation angle pitch, which is one of the imaging system geometry parameters, is set to 1.33 ° (= 200 ° / 150 sheets), for example.
(Step 403) A rotation center axis position (hereinafter referred to as center) is set to an initial position (design position).
(Step 303) Rotational orbit plane image reconstruction is performed based on projection data of the first half of the rotation of the wire phantom.

  Here, FIG. 7 and FIG. 8 are schematic diagrams showing what kind of reconstructed image is obtained when the initial position value of the rotation center axis has a mechanical manufacturing error. However, FIG. 7 and FIG. 8 are cases where the number of wires of the wire phantom 30 is 3 × 3. 7 and 8 are viewed from the direction of the foot of the subject 3. For simplicity, only the X-ray image intensifier 5 and the TV camera 6 are shown, and the others are omitted. Since what is now being described is the first half of the rotation, the X-ray image intensifier 5 and the TV camera 6 move from the direction of the left hand of the subject to the direction of the ceiling. FIG. 7 is a reconstructed image when the image reconstruction processing is performed on the assumption that the projection of the rotation center axis on the detector has an error in the + u direction from the actual position (32 solid line positions). In this case, the wire image of the reconstructed image is not a point, but an arc consisting of an envelope is drawn from the right to the bottom, and its radius is approximately equal to the deviation from the correct center position set in (Step 402). . On the other hand, FIG. 8 shows a reconstructed image when the image reconstruction process is performed on the assumption that the projection of the rotation center axis on the detector has an error in the −u direction from the actual position (32 solid line positions). In this case, the wire image of the reconstructed image does not become a point, but draws an arc of an envelope from the left to the top, and its radius is approximately equal to the deviation from the correct center position set in (Step 402). .

(Step 404) Threshold processing is performed on the reconstructed image obtained in (Step 303) to extract a wire region.
(Step 410) In the reconstructed image, a plurality of wire cross sections are arranged vertically and horizontally, but in the rotation center axis position calculation process, attention is paid to the wire arranged most near the center on the image. And the radius of this center wire is calculated.
(Step 411) It is determined whether or not the arc radius calculated in (Step 410) can be regarded as zero. If it can be regarded, it moves to (step 420), and if it cannot be regarded, it moves to (step 412).
(Step 412) When the radius of the arc cannot be regarded as zero in (Step 411), it is determined whether it is the case of FIG. 7 or FIG. 8, and correction is performed by adding / subtracting so that the center becomes correct accordingly. Then, the process from (Step 303) is performed again. If the arc radius can be regarded as zero in (Step 411), the process proceeds to calculation of the rotation angle pitch.

  9 and 10 show reconstructed images of the wire phantom when the midplane reconstruction is performed at a rotation angle pitch different from the actual one. However, FIG. 9 and FIG. 10 show the case where the number of wires of the wire phantom 30 is 3 × 3. FIGS. 9 and 10 are viewed from the foot direction of the subject 3. For simplicity, only the X-ray image intensifier 5 and the TV camera 6 are shown, and the others are omitted. FIG. 9 is a reconstructed image when the midplane reconstruction is performed with the rotation center pitch being correct but the rotation angle pitch being smaller than the actual value. In this case, on the line passing through the center of rotation and passing through the middle direction of the scan angle range, in the figure, the upper right, center, and lower left wire cross-sections are imaged, but as they move away from the line, the wire cross-section spreads outward. Draw an arc. On the other hand, FIG. 10 is a wire reconstructed image when the midplane reconstruction is performed with the rotation center pitch being correct but the rotation angle pitch being smaller than the actual value. In this case as well, although the wire cross section is imaged on a line passing through the center of rotation and passing through the middle direction of the scan angle range, an arc surrounding the center of rotation is drawn outside the wire cross section, contrary to FIG.

(Step 420) The radius of the arc of the reconstructed image of the peripheral wire, that is, the wire farthest from the line passing through the rotation center and passing through the middle direction of the scan angle range, in the figure, the upper left or lower right wire is calculated.
(Step 421) It is determined whether or not the arc radius of the peripheral wire calculated in (Step 420) can be regarded as zero. If it can be considered, the reconstructed image becomes as shown in FIG. 11, and the calculation of the rotation center axis position and the rotation angle pitch in the first half of the rotation is finished. (However, in FIG. 11, the cross section of the reconstructed image of the wire is a slanted rod and does not form a dot because the projection data from the upper left to the lower right is missing.)
(Step 422) If the radius of the arc cannot be regarded as zero in (Step 411), it is determined whether it is the case of FIG. 9 or FIG. 10, and correction is performed by adding / subtracting so that the rotation angle pitch is correct accordingly. . Then, the process from (Step 303) is performed again.

  The rotation center axis position and the rotation angle pitch are calculated for the second half of rotation by the same flowchart. Then, a wire reconstruction image as shown in FIG. 12 is obtained. If the geometric parameters of the first half of rotation and the second half of rotation are the same, the addition of the reconstructed images in FIG. 11 and FIG. Since the geometric parameters of the first half and the second half of the rotation are different, overlay processing such as rotation and translation is necessary when adding the reconstructed images. This is performed based on the geometric parameters obtained by the matching reference position calculation means 305.

Next, FIG. 14 shows a flowchart of a process for obtaining a geometric parameter necessary for superimposing the reconstructed images of the first half of the rotation and the second half of the rotation in the rotation start angle and superimposition reference position calculation unit 305. Each step will be described in turn. First, in the wire image reconstruction in the first half of the rotation, adjustment is performed so that the wire cross section is horizontally arranged. Specifically, the wire cross section is made horizontal by adjusting so that the rotation start angle direction in the reconstruction calculation is actually matched.
(Step 501) First, the rotation start angle direction in the reconstruction calculation is set to the design value −100 °.
(Step 502) A rotational trajectory plane reconstructed image as shown in FIG. 11 is generated from the projection data in the first half of the rotation.

(Step 503) A wire region is extracted from the reconstructed image by threshold processing.
(Step 504) The barycentric coordinates of each wire cross section are calculated from the wire region extracted in (Step 503).
(Step 505) It is determined whether or not the reconstructed cross section of the wire is correctly placed horizontally, that is, the vertical coordinates of the center of gravity of the wire cross section aligned in the lateral direction are equal. Go to step 506).
(Step 506) When it is determined in (Step 505) that the vertical coordinates of the centroids of the wire cross sections arranged in the horizontal direction are not equal, the first half rotation start angle direction in the reconstruction calculation is added or subtracted according to the unequal degree, The process from step 502) is performed again.

  If it is determined in step 505 that the vertical coordinates of the center of gravity of the wire sections arranged in the horizontal direction are equal, the wire section is horizontally arranged in the wire reconstruction image in the latter half of the rotation, and the center of gravity is The process proceeds to the calculation of the angular direction of the latter half of the rotation and the overlapping reference position so as to coincide with the constituent images.

(Step 510) First, the registration reference position in the latter half of the rotation is set as designed (no deviation between the first half of the rotation and the second half of the rotation).
(Step 511) Furthermore, the start angle of the latter half of the rotation is set to 0 ° in the ceiling direction.
(Step 512) A midplane reconstructed image as shown in FIG. 12 is generated from the projection data in the latter half of the rotation.
(Step 513) A wire region is extracted from the reconstructed image by threshold processing.

(Step 514) The barycentric coordinates of each wire cross section are calculated from the wire region extracted in (Step 513).
(Step 515) It is determined whether or not the reconstructed cross section of the wire is correctly arranged horizontally, that is, the vertical coordinates of the center of gravity of the wire cross section aligned in the lateral direction are equal. Move to step 516).
(Step 516) If it is determined in (Step 515) that the vertical coordinates of the centroids of the wire sections aligned in the horizontal direction are not equal, the second half rotation start angle direction in the reconstruction calculation is added and subtracted, and the process from (Step 512) is performed. Try again.
(Step 520) If it is determined in Step 515 that the reconstructed cross section of the wire in the latter half of the rotation is accurately placed horizontally, the reconstructed cross section of the wire in the first half of the rotation and the reconstructed cross section of the wire in the second half of the rotation Overlapping.

(Step 521) It is determined whether the wire centroid coordinates of the reconstructed cross section in the first half of rotation and the wire centroid coordinates of the reconstructed cross section in the second half of the rotation are coincident with each other. If the calculation is terminated and cannot be regarded, the process proceeds to (Step 522).
(Step 522) If it is determined in (Step 521) that the wire centroid coordinates of the reconstructed cross section in the first half of rotation and the wire centroid coordinates of the reconstructed cross section in the second half of the rotation are not coincident, they are overlapped by the deviation of the centroid coordinates The process from (Step 512) is performed again after correcting the reference position.

Next, FIG. 15 shows a flowchart of processing for calculating the coordinate twist angle in the coordinate twist angle calculation means 306. Each step will be described in turn.
(Step 601) First, the initial value of the coordinate twist angle is set to 0 °.
(Step 602) Using the geometric parameters obtained by each of the above means, a three-dimensional image reconstruction process is performed not only from the rotating orbital plane 7 but also from all wire projection data.

(Step 603) A reconstructed cross section that is parallel to the rotating track surface 7 and is separated from the rotating track surface 7 in the direction of the rotation center axis is selected. As can be seen from FIG. 2, if the coordinate torsion angle b is not zero, even if a reconstructed image is formed on the u-plane of the rotating orbital plane 7, as the reconstructed cross-section moves away from it, Since the axis 22 deviates from the projection 21 of the rotation center axis, the wire cross section of the reconstructed image draws a locus as shown in FIG. 7 or FIG.
(Step 604) A wire region is extracted from the reconstructed image by threshold processing.
(Step 605) The radius of the arc of the wire is calculated.

(Step 606) It is determined whether the radius of the arc of the wire can be regarded as zero. If the radius of the arc of the wire can be regarded as zero, the coordinate torsion angle calculation process is terminated, and if not, the process proceeds to (Step 607).
(Step 607) When the radius of the arc of the wire cannot be regarded as zero in (Step 606), the value of the coordinate twist angle is increased or decreased according to the direction of the arc, and the process from (Step 602) is performed again.

  According to the embodiment described above, in the C-arm type cone beam X-ray CT apparatus, when there is a bias in the geometric parameter representing a mechanical manufacturing error or the like due to the rotating angle range, the wire phantom is used in each angle range. The geometric parameters are determined by each means, and the 3D X-ray CT image is reconstructed using the geometric parameters determined by each means, and the 3D X-ray CT images obtained in each angle range are suitable. Therefore, it is possible to suitably obtain and correct the above-mentioned error that causes artifacts.

  The present invention is not limited to the above embodiment, and can be variously modified without departing from the gist of the present invention. For example, in the above embodiment, the case where the geometric parameters are different between the first half of the rotation and the second half of the rotation is shown, but it is also necessary for a C-arm type cone beam X-ray CT apparatus having three or more geometric parameters with different angle ranges. The present invention can be applied by adding various steps. In addition, a system composed of an X-ray image intensifier 5 and a TV camera 6 was used as a two-dimensional X-ray detector. For example, this X-ray detector was replaced with a two-dimensional X-ray detector using a TFT element. Even in this case, the present invention is applicable. The present invention can also be applied to a cone beam X-ray CT apparatus that is not a C-arm system. In the above-described embodiment, the rotation angle is 200 degrees. However, the present invention can be applied to other angles, for example, when the entire angle is 360 degrees.

1 is a block diagram showing a schematic configuration of a C-arm type cone beam X-ray CT apparatus in an embodiment of the present invention. The figure for demonstrating a coordinate twist angle. The figure for demonstrating the phantom used in order to calculate a geometric parameter in the geometric parameter calculation means 300 (when arrange | positioning a wire 5 * 5 in length). The flowchart which shows the procedure of the geometric parameter calculation process in embodiment of this invention. The flowchart which shows the procedure of the rotation track surface reconstruction calculation process in embodiment of this invention. The flowchart which shows the procedure of the rotation center axis position calculation and rotation angle pitch calculation process in embodiment of this invention. Schematic diagram of what kind of reconstructed image is obtained when the initial value of the rotation center axis has a mechanical manufacturing error (assuming that the projection of the rotation center axis on the detector has an error in the + u direction from the actual value) When image reconstruction processing is performed). Schematic diagram of what kind of reconstructed image is obtained when the initial value of the rotation center axis has a mechanical manufacturing error (the projection of the rotation center axis on the detector has an error in the -u direction from the actual As an image reconstruction process) Reconstructed image of the wire phantom when reconstructing the rotating track surface at a different rotation angle pitch (when the rotating track surface is reconstructed with a smaller rotation angle pitch). Reconstructed image of the wire phantom when reconstructing the rotating track surface at a different rotation angle pitch (when the rotating track surface is reconstructed with a smaller rotation angle pitch). FIG. 6 is a schematic diagram showing a reconstructed image when the rotational orbital plane is reconstructed from the projection data of the first half of rotation in the present embodiment, with the rotational center axis position and rotational angle pitch being geometrically correct. In the present embodiment, a schematic diagram showing a reconstructed image when the rotational orbital plane reconstruction is performed from the projection data in the latter half of the rotation with the rotational center axis position and the rotational angle pitch being geometrically correct. In this Embodiment, it is a schematic diagram which shows the reconstruction image when the position of a rotation center axis | shaft and the rotation angle pitch are reconfigure | reconstructed with a geometrically correct value from all projection data. The flowchart which shows the procedure of the process which calculates | requires a geometric parameter required when the reconstructed image of the rotation first half and the rotation latter half are overlapped. The flowchart which shows the procedure of the process which calculates a coordinate twist angle.

Explanation of symbols

304 ... Rotation center axis position and rotation angle pitch calculation means
305 ... Rotation start angle and overlay reference position calculation means
306 ... Coordinate torsion angle calculation means

Claims (5)

  The X-ray of the subject arranged on the rotation center axis while rotating the X-ray source and the two-dimensional X-ray detector arranged opposite to the X-ray source on the circular orbital surface having the same rotation center. Cone beam X-ray CT apparatus for generating and displaying a three-dimensional X-ray CT image by capturing a fluoroscopic image with the two-dimensional detector and performing image reconstruction calculation based on the captured X-ray fluoroscopic image In this case, the error of the rotational center axis of the rotational movement and the difference between the rotational start angle and the rotational speed set value of the rotational movement are corrected for a phantom including a plurality of wires arranged substantially parallel to the rotational center axis. A cone beam X-ray CT apparatus, comprising: error calculation means for calculating using a constituent image; and correction means for correcting the three-dimensional X-ray CT image using the calculated error.   Each of the errors corresponds to the first rotation angle range and the second rotation angle range, and is calculated by the error calculation unit and calculated corresponding to the first rotation angle range. The error is used to correct the three-dimensional X-ray CT image corresponding to the first rotation angle range by the correction means, and the error calculated corresponding to the second rotation angle range is the correction means. Is used to correct the three-dimensional X-ray CT image corresponding to the second rotation angle range, and calculates the positional deviation between the corrected first and second three-dimensional X-ray CT images. And a superimposing unit that superimposes the three-dimensional X-ray CT images in the first and second rotation angle ranges by correcting based on the calculated misregistration. The cone beam X-ray CT apparatus according to claim 1.   3. The cone beam X-ray CT apparatus according to claim 2, wherein the number of divisions of the angle range for dividing each error is three or more.   Means for calculating an angle (hereinafter referred to as a torsion angle) between a direction of a projection line of the rotation center axis to the two-dimensional detector and a direction of the projection line designed for the two-dimensional detector; 4. The cone beam X-ray CT apparatus according to claim 1, further comprising means for correcting a twist angle of the X-ray fluoroscopic image using the calculated twist angle.   The phantom including a plurality of wires arranged substantially in parallel is arranged so that the plurality of wires are arranged vertically and horizontally when viewed from a cross section perpendicular to the direction of the wires.

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